The invention concerns circuitry for reversing a magnetic field.
Circuitry for this type is employed for example in magneto-optical recording- and -playback equipment to reverse the magnetism in the magnetic layer of magneto-optical software.
A known magneto-optical software is the magneto-optical disc, at which a magneto-optical layer is behind a light-transmissive layer, on the magneto-optical layer information can be recorded and recalled from. How the information is recorded on a magneto-optical disc will now be described.
A laser beam is focused on the disc and heats the magneto-optical layer to a temperature in the vicinity of its Curie point. It is, however, usually sufficient to heat the layer only to approximately its compensation temperature, which is below the Curie temperature. An electromagnet is positioned beyond the focal point on the disc and magnetizes the area heated by the laser beam in one direction or the other. Since the heated area cools down to below the compensation temperature again once the laser beam is turned off, the magnetic orientation by the electromagnet is retained. It freezes in, so to speak. The individual bits are accordingly stored in domains of different magnetic orientation. One orientation corresponds for example to a domain of logical ONE and the other to a domain of logical ZERO.
The Kerr effect is exploited to recall the information. The plane of polarization of a linearly polarized beam of light is rotated while being reflected in a magnetized mirror around a measurable angle. The plane of polarization of the reflected beam of light is rotated right or left in accordance with the magnetic orientation of the mirror. Since, however, the individual domains on the disc act like magnetized mirrors, the plane of polarization of a beam of light that scans the domains will be rotated right or left around a measurable angle in accordance with the magnetic orientation of the domain just scanned.
From the angle of rotation of the plane of polarization of the beam of light reflected from the disc, an optical pick-up can determine whether the bit that is present is a ONE or a ZERO.
One known solution for magnetizing the magneto-optical layer in one direction or the other provides a circuit arrangement that acts as an electromagnet with a coil behind the magneto-optical disc. The coil is to be of such a size that it has the capacity to remagnetize the entire area scanned by the optical pick-up means. This area will be, depending on the type of recording and playback equipment, for example, a radial or circular-segmental strip which extends from disc edge to disc center. As the field strength must attain a minimum value over the entire strip in order to remagnetize the strip, the cross-section and therewith the inductivity will be relatively large.
In another known solution, the coil is affixed to the optical pick-up means. The coil can be wound around the objective lens of the optical pick-up for example. Owing to the fact that in this solution the coil together with the optical pick-up is guided along the data tracks on the magneto-optical disc by means of a tracking circuit, a smaller cross-section and therefore a smaller inductance are sufficient for generating the same minimum field strength because it is not a radial or circular-segmental strip but rather only a small, for example circular-shape area with the virtually point form laser spot as center that needs to be remagnetized in the magneto-optical layer.
A circuit arrangement for the rapid reversal of a magnetic field is known from GB-A-2 184 625.
A series circuit consisting of a first and a second diode, and a series circuit consisting of a third and a fourth diode as well as a direct voltage source, are connected parallel to each other. The common terminal of the first and the second diodes is connected via a coil with the common terminal of the third and fourth diodes. Each of the first through fourth diodes, all of which are polarized in the blocking direction to the direct voltage source, is bypassed by a controllable switch. A fifth diode, which is polarized in the transmitting direction to the direct voltage source, is located between the common terminal point of the first and third diodes and the direct voltage source. A capacity is provided parallel to the direct voltage source.
Although this circuit arrangement achieves a rapid reversal of the coil's magnetic field, the time taken to reverse the magnetic field can be even further reduced by the use of appropriate measures.
It is therefore the object of the invention to set up a circuit arrangement according to the preamble of claim 1 so that a reliable and quick reversal of the magnetic field is effected.
The invention solves this task by means of the features given in claim 1.